This invention relates to piezoelectric materials and, more particularly, to fabrication of polyvinylidene fluoride (PVF.sub.2) including unoriented phase I crystallites which has desirable piezoelectric characteristics for it to be used in acoustic transducers.
Since the initial discovery of the large piezoelectric response of poled, oriented films of PVF.sub.2 by Kawai in 1969, several subsequent studies have been made to elucidate the mechanisms responsible for this phenomenon. Since the greatest piezoelectric activity is found for PVF.sub.2 films with the phase I crystal structure, it is desirable to find a satisfactory explanation to account for this feature. Mechanisms most often discussed are: a bulk polarization of the sample due to field induced reorientation or switching of the molecular dipoles in the polar phase I crystals; non-uniform charge injection leading to an asymmetric distribution of real charge in the sample; a field induced charge in trapping of injected or ionic charges present as impurities; and some type of surface phenomenon caused by the strong interaction between the positive electrode and the film during poling. Regardless of the mechanism accounting for the piezoelectric activity of PVF.sub.2 in the phase I crystal structure, it is desirable to have a simple technique of fabricating this material to exploit it for use in acoustic transducers.